Without limiting the scope of the present invention, its background will be described in relation to perforating a wellbore, as an example.
After drilling the various sections of a subterranean wellbore that traverses a formation, individual lengths of relatively large diameter metal tubulars are typically secured together to form a casing string that is positioned within the wellbore. This casing string increases the integrity of the wellbore and provides a path for producing fluids from the producing intervals to the surface. Conventionally, the casing string is cemented within the wellbore. To produce fluids into the casing string, hydraulic openings or perforations must be made through the casing string, the cement and a distance into the formation.
Typically, these perforations are created by detonating a series of shaped charges that are disposed within the casing string and are positioned adjacent to the formation. Specifically, one or more charge carriers or perforating guns are loaded with shaped charges that are connected with a detonator via a detonating cord. The charge carriers are then connected within a tool string that is lowered into the cased wellbore at the end of a tubing string or other conveyance. Once the charge carriers are properly positioned in the wellbore such that the shaped charges are adjacent to the formation to be perforated, the shaped charges may be fired. If more than one downhole zone is to be perforated, a select fire perforating gun assembly may be used such that once the first zone is perforated, subsequent zones may be perforated by repositioning and firing the previously unfired perforating guns without tripping out of the well.
Typically, oil well perforating operations involve a thorough check of the perforating gun system or gun string. The operator must ensure the system is electrically robust as well as safe. A typical perforating operation involves a “check fire test” where the operator verifies the surface system as well as the downhole equipment, usually involving a casing collar locator and a cable head. The purpose of this check is to verify that there are no insulation leaks as well as to verify the electrical continuity of the whole system. There are no explosives involved in the check process.
Subsequently, the oil well perforating operations may connect the one or more explosive devices to the already checked casing collar locator plus cable head. The explosive device may then be armed. At this point, all electrical sources are shut down and the logging cable is shorted at the surface. The procedure requires that all the electrical sources can be restored when the device is below 200 ft. from ground level. No further test on the explosive device electrical continuity, however, can be performed on an armed device. A regular blast meter could be used after the explosive device is below 200 ft., but such operation is not allowed as it requires the operator to manually access the wireline circuit at the surface with the associated risk of making a bad connection or using the wrong type of meter, which could risk unintentional explosion initiation out of the intended depth.